ML20090L621

Preliminary Rept of Fuel Cask Drop Accident Analysis

ML20090L621
Person / Time
Site:	Monticello
Issue date:	10/01/1974
From:	NORTHERN STATES POWER CO.
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ML20090L617	List: ML20090L615 ML20090L621
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NUDOCS 9102120460
Download: ML20090L621 (8)

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9 NORTilERN STATES P0k'ER CQiPANY MONTICELLO NUCLEAR GENERATING PLANT REPORT TO Tile LHITED STATES ATQ11C ENERGY COMMISSION DIRECTORATE OF LICENSING Docket No. 50-263 License No. DPR-22 PRELIMINARY REPORT OF FUEL CASK DROP ACCIDENT ANALYSIS Dated October 1, 1974 9102120460 PDR 741001 P

ADOCK 05000263 PDR

)

• INTRODUCTION l

We Regulatory Staf f of the Directorate of Licensing, USAEC, is currently considering possible accident conditions and postulated events which result from a spent fuel shipping cask drop.

A detailed assessment of the effects of a spent fuel shipping cask drop j accident for the Nonticello plant involved the determinstion of possible l plant damage incurred including:

Spent Fuel Pool Integrity l

Spent Fuel Integrity '

Safety of Vital Equipment and Systems Cask Lifting Systems Operations and Safety The structural, mechanical systems and cask handling equipment at the Monticello ,

plant have been subjected to a detailed safety evaluation in response to the j subject AEC request for infomation (1). A sunanary of the results of these evaluations are presented in this preliminary report.

For a description of the cask used in this analysis refer to reference (6).

SPENT FUEL POOL INTEGRITY The fuel sterage and handling systems are described in Section 10.2.1 of the Monticello FSAR. The pool and its support systems are designed to Seirmic Class I conditions. {

An analysis was conducted to detemine possible structural damage to the spent fuel pool floor that could resu!*. from dropping a fuel shipping cask from various

• heigh t s. The method used W uer .rmine the impact loading of the dropped cask ,

can be found in reference (2). Once the impact loading was determined, standard beam deflection methods were ut'd to~ evaluate the effects on the structures. The effects of buoyancy and friction were taken into consideration in the analysis, i W e FSAR analysis of the cask drop accident, using the fuel shipping cask design envisioned at that time, demonstrated that minor cracking of the fuel pool would result. Based upon the results of a conservative analysis of the spent fuel pool floor structure, using the IF-300 shipping cask, caly minor cracking cannot be assumed from all possible drop heights, t

SPENT FUEL INTEGRITY l Fuel cask handling precautions have been imposed in the Monticello FSAR which require that the shipping cask not be moved over the reactor vessel or spent fuel in the storage pool. Furthermore, the cask will not be lifted above the spent fuel pool except in the designated area above the cask laydown pad on the fuel pool floor.

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In the event that the spent fuel cask woro to be tippod over or deflected into  !

i . the fuel pool, the cask size is adequate to reach spent fuel stored in close  !

] proximity to the cask laydown pad. However, a cask drop on the pool edge [

is expected to spali the impact area and force the cask into the pool nearly '

i vertically. Thus, a horizontal cask drop is considered highly unlikely due l to the mass and size of the cask. Direct fuel damage from a cask drop is i

thereby ifmited to fuel stored in close proximity to the cask laydown pad. '

INTEGRITY OF CRITICAL SYSTEMS AND COMPOND4TS

An assessment has been conducted to determine the mechanical equipment and j systems affected by a cask drop while being transported across the reactor

, building operating floor or while being lif ted in the building equipment ,

j hatch from the rail shipping car. By means of a structural analyets i identical to that used for the spent fuel Pool floor it can be concluded i that, with the use of proper administrative controls, the reactor butiding

! uperating floor has sufficient structural strength to withstand a fuel cask j drop while sustaining only minor damage. All equipment and systems below '

2 the reactor building operating floor would be protected from damase in *

} this case.

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A cask drop from the ma> nium drop height in the equipment hatch area could cause structural and possibly cask damage. Cask handling procedures are

being evaluated to provide adequate protection to plant structures and j equipment in this area, J

! PESCRIPTION OF Tils flAlWLING EQUllND4T TOR Tile SPD4T FUEL CASK

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! REACTOR BUILDING CRANE

.I 1

The Reactor Building Crane is an indoor, electric powered traveling bridge I crane, with a single trolley cab or pendent controlled 85 ton main hoist. The l crane was censtructed by the Crane Manufacturing and Service Corporation to i the requiruments for a Class A crane established by the Electrical Overhead Crane Institute Specification 61 (3), and Bechtel Design Specification 5828 M-3 (4).

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{ The Reactor Building Crane was designed to the following safety factors, i

i COMP 0NENT RAFETY FACTOR  !

] Main Hoist 6 Cables 6 i i Main Hook 6 for gansion in stem I

5 for combined bending and tension 1

Welds 5 based on ultimate

strength of metal in

welds 1

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4 MAIN HOIST DRIVE i

1he main hoist motor is a shur t wound, DC inotor, rated at 40 h.p. In addition i

to the shunt field, the motor has a separate series vinding for use during dynamic braking, power for the hoist motor is ppplied by an AC driven motora generator set installed on the crene. The generator is a shunt wound DC machine, lhe hoist motor is controlled by a General Electric Maxspeed 320 Crane Drive The drive unic varies motor field and armature cvrrents to control the

~

unit.

speed and r:,rque of the motor along a constant horsepower char 0.cteristic, consequently, the speed of the hoist motor is a function of the weight of the load being lifted. At full load the hoist motor rated speed it 850 rpm which

! is equivalent to a drum speed of 5 fpm. Nith an empty hook, the hoist motor j speed is approximately 2720 rpm which is equivalent to a drum speed of 16 fpm.

These are maximum speeds for their load condition. However, the crane operator j may vary the speed of the hoist from a creep rate pp to these maximum speeds.

BRAKING i

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Mechanical braking is provided by two solenoid operated disc and shoe brakes on the main hoist, one installed on the free end of the hoist motor and one on the gear reducer input shaft. The brakes are spring loaded and automatically set whenever electrical power is removed from the armatures. The brakes are released (armature energized) simultaneously with the energitation of the main 2

hoist motor. The brakes are energized by direct current from a static exciter /

i transformer system off the input power to the main hoist motor-generator set.

Each brake is designed to hold 1507 rated full load hbist torque at base speed.

Regenerative braking is used to provide control braking when lowering heavy loads and to decelerate the motor from fast speeds before applying the mechanical brakes. The hoist motor drive ur.it controls the amount of braking by means of a polartty reversing power amplifier in the shunt field of the generator.

Dnergency braking actf on is effected by inanediate application of both mechsnical holdins brakes when: (1) the hoist motor voltage decreases to 70 volts, (2)

, upper or lover hoist travel limit switches are activated or (3) upon loss of power to the main hoist motor. In addition, dynamic braking is provided to assist the mechanical brakes in the event regenerative braking is unavailable due to a power failure. Upon interruption of power, the series field is auto-matica11y inserted in the motor DC power loop, driving the motor as a self-excited generator. 1he generated current is dissipated in a load resistor providing mechanical braking torque. Although dynamic braking will prevent free-fall of the load during an emergency, operation of one toechanical brake is necessary to bring the load to a stop. Based upon an anlysis of the dynam:c braking capability for the motor-drive system, load descent speed under thfa condition would be limited to 10.5 feet per minute. Fuel cask descent at this speed to the floor levels would not result in loss of floor structural integrity.

E1 HIT SWITCHES There are three main hoist travel limit switches, two for upper-hoist-travel and one for lower-hoist travel. One of the two upper-hoist-travel limit switches is located on the top block assembly and is activated by the cable.

'The other upper hoist trevel limit switch is directly couplod to the hoist drum 1 -

and is activated by drum rotation. Each upper hoist-travel limit switch de-j energises a mechanical brake and the hoisting motor.The upper hoist travel limit switch electrical control circuit is designed so that both mechanical brakes and hoist motor will be deenergised by one limit switch on loss of one limit switch. The lower hoist travel limit switch is directly coupled to i the hoist drum and is activated by drum rotation. The lower hoist-travel limit switch

de-energises both mechanical brakes and the hoisting motor when the main hook 4 is three feet above the ground floor elevation.

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HOIST MECitANISMS l A four series gear reducer decreases the hoisting motor speed to the drum

, speed by a ratio of 139 to 1. The first reduction gear and pinion is a herring-1 bone gear arrangement and the remaining reduction gears and pinions are spur gears.

4 The hoisting drum is constructed of all welded steel. The drum is machine j threaded with righthand and lef t hand cable grooves and is flanged to prevent the cable from slipping of f the drum. The diameter of the drum was designed so that two complete wraps of cable remain on the drum when the hook is in its lowest position. The cable is a 12 part 6x37 improved plow steel wire with a hemp core. The main hook is a twin sister type with safety latches. A six inch diameter shackle hole is provided in the center of the hook. This shackle

} hole is used for lif ting the spent fuel cask. The hook will remain fixed in

any selected direction during lowering or raising.

The spent fuel shipping cask lifting yoke is a steel structure which consists of two yoke legs, two cross members, and a retractable six inch diameter heat treated pin for insertion in the main hook shackle hole. The two yoke legs engage the cask lif ting trunnions, the six inch diameter pin is inserted through the two yoke leg crossmembers and the main hook shackle hole. The cask lif ting yoke is designed for a minimum load safety factor of 3.

CRANE BRIDGE AND TROLLEY 1he bridge of the crane consists of two welded box section girders rigidly attached to the bridge trucks. The bridge drive consists of two 7.5 h.p.

direct current motors each coupled to a drive wheel axle through a gear reducer.

The drive systems are located on opposite sides of the box girders. The drive

motors receive electrical power from their own motor-generator set. The bridge drive ha s regenerative braking similar to that described in the Main Hoist section.

1he bridge parking brake is a solenoid operated disc and shoe braka which is set and released as described in the Main Holst section for the mechanical brakes.

The parkicg brake is designed to hold 150% of the rated full load drive motor torque and sots Jatamatically on loss' of power to the brake or drive motor. The

,truch wheels are deuble flanged machine steel. The end truck has safety lugs to Ilmit the drop of the truck in the event of a wheel or axle failure. The bridge b r two endrof-travel Ilmit switches which, when activated, de-energize the drive motors and brake solenoids. The bridge has four polyurethane bumpers i

and runway stops to prevent over-travel of the bridge, i

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'ne crane trolley connists of a rigid, braced, box steel frame attechod to tho l We trolley drive consists of a 2 h.p. direct current motor, a j trolley trucks.

gear reducer and two drive shaf ts, one to each drive wheel, he trolley drive J 'notor receives electric power frotn its own motor-generator set. W e trolley

drive motor has regenerative braking as described in the Main Hoist section.

n e trolley has two solenoid operated disc and shoe brakes which are set and ,

i released as described in the Main Hoist Mechanical Braking section. We i solenoids are de energised to engage the brakes. Each parking brake is de-signed to hold 507. of the rated full motor torque and sets automatically on +

j toss of pcver to the brakes or trolley drive motor, n e trolley truck wheels j 4

are double flanged machine steel. The trolley trucks have safety lugs to limit j the drop of the trolley in the event of a wheel or axle failure. We trolley

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movement is controlled by an end of travel liinit switch, which when activated j de energines the trolley drive motor and brake solenoids. We trolley has j j polyurethane bumpers to prevent over travel. l

,QUA1,IpICATION TESTING  ;

l Pre service gaalification tests were conducted on the reactor building crane 1 during " Pre operational Test No. B-22," March 21, 1970. All limit switches on  :

l the book travel, trolley travel and bridge travel were tested. The l:rolley ,

l and bridge brakes and speed controls were checked. The pendant controle and l deadman switch were checked. We main hook was load tested to the maximum l

rating of the crane (85 ton). The deflection of the bridge at rated load was checked. The mechanical holding brakes were slip tested at rated load. We j main hook descent speed at rated load was verified to be 6 feet per minute.

J Subsequent to load testing, the main hook was Magnafluxed. Additional tests and inspections are conducted in accordance with periodic maintenance and operating l procedures. Section 10.2.1 of the Monticello FSAR also describes testing  !

j activites planned prior to spent fuel cask handling operations. L i '

1 A sunanary table of reactor building crane design parameters as specified by j Reference (4) and the Crane Manufacturing and Service Corporation design manual l (5) is given in Table 1. A corebination of proven engineering practices l

in the design of the crane and the implementation of special precautions as I reported in Section 10.2.1.3 of the Monticello FSAR, to avoid dropping of heavy I objects insures crane safety and reliability will be maintained during fuel cask  ;

l handling operations. ,

i l CONCI,USIONS l

I i

! n e results of our analysis of the cask drop accident, while it cannot demonstrate i absolute assurance of only minor structural damage in the postulated cask drop, '

does indicate that the combination of proper administrative controls and the  ;

) conservatively rated and highly reliable handling equipment do provide reasonable l

assurance to preclude the cask drop event, i

. His subject will be a matter of further detailed review and consideration by NSP when fuel shipping plans and schedules are determined and firm decisions are made as to selection of the cask to be utilised for fuel shipping.

A final report on this subject will be submitted, demonstrating that spent fuel l

> handling operations can be safely conducted at Monticello, in sufficient time to '

i prevent disruption of the first anticipated shipment.

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1 TABLE 1 MONTICELLO REACTOR BUILDING CRANE

, DESIQ1 PARAMETER

SUMMARY

a

]

1 i Parameter Main Holst Auxiliary Holst I

I Load Capacity 85 ton 5 ton i

control Station Trolley Cab pendant Design Speed

Full Load 5 fpm 20 fpm No Load 16 fpm j Lifting Range 117'-0" 117'-0" Drive Motor 40 h.p. (240VDC) 7.5 h.p. (240VDC)

Type of Braking Systems: Holding / Regenerative / Dynamic uchan. cal (Holdt.ig) Brakes: 2 disk and shoe >

i Capacity 150% of motor torque 1

Crane Span 98'-9" 3

Length of Runway 130'-6" Cable 12 part 6 x 37 plowsteet 4

Hook Twin (sister) with 6" shackle hole Hook Limit Switches 2 upper-hoist-travel I lower-hoist-travel Motor-Generator Set 50 h.p. (230/240VAC)/33KW 4 Trolley Design Speed 10fpm Tro116/ Drive Motor 2 h.p. (240VDC) 1 Trolley M-G Set 3 h.p./1.5 KW Bridge Design Speed 50 fpm Bridge Drive Motors 2-7.5 h.p. (240VDC)

Bridge M-C Set 20 h.p./13 KW ,

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RETERENCES

1. D. J. Skovholt, letter to L. O. Mayer, Director of Nuclear Support Services Northern States Power Company from U.S. Atomic Energy Con-mission, Directorate of Licensing, Re License No. DPR-22, February 4,1974.

2. R. J. Roark, " Formulas for Stress and Strain," McGraw Hill Book Company, 4th Edition, p. 370, 1965,

3. Electric Overhead Crane Institute, " Specification No. 61 for Electric l Overhead Traveling Cranes," One Thomas Circle, N.W. , Washington D.C.

j

4. Bechtel Corporation, " Specification No. 5828-M-3 for Reactor Building j Bridge Crane for the Monticello Nuclear Generating Plant," San Francisco, j Revision 2, March 29, 1968.

i j 5. Crane Manufacturers Service Corporation, " Design Manual 5828 M3-41-1,"

4 Crane Serial No. 514, 85/5 Ton Powerhouse Crane for Monticello Nuclear l Plant, November 4,1968.

6. General Electric Company, " Technical Description Ir 300 Irradiated Fuel j Shipping Cask," report No. NEDO-10864, July 1972.

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_AEC DISTRIBUTION FOR PART 50 D_0_CKE T 6%TERI AL (TEMPORARY FORMI CONT ROL NO'._104 52 j FILE: -

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XXX 40 $0-263 DESCRIPTION: ENCLOSURES:  ;

Ltr trans the following... Analyses of the Spent Pool Shippian Cask Drop Acc ident. . . . . .

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